Serial plastic sections were cut from brains of rats that had received horseradish peroxidase (HRP) and lanthanum chloride (La) after infusion of hyperosmotic arabinose into the internal carotid artery. In a few junctions between endothelial cells of cerbral vessels, this smallest electron dense tracer, ionic lanthanum, could be followed from the vessel lumen to the perivascular basal lamina, a continuity indicating deformation of the junctions due to shrinkage of the endothelial cells. HRP did not appear to enter in the fashion. In order to see whether FRP passes across endothelium and choroid plexus epithelium actively, by vesicular transport, or passively, i.e., extracellularly via junctions, the temperature of the brain was reduced to 22-24 degrees C by hypothermia. In these brains, the number of HRP-laden vesicles in the endothelium have yet to be evaluated, but the number in the choroidal epithelium was very low, a decrease indicating in vivo, that protein uptake is due to an active endocytosis and can be diminished or halted at low temperatures.